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Calibrating radar sounder instruments through correlated observations of solar radio burst events: preliminary results with SHARAD and STEREO

Presentation #118.06 in the session Mission-supporting Practices, Modeling, and Data (Poster + Lightning Talk)

Published onOct 23, 2023
Calibrating radar sounder instruments through correlated observations of solar radio burst events: preliminary results with SHARAD and STEREO

Recent research has shown that the Shallow Radar (SHARAD) instrument onboard the Mars Reconnaissance Orbiter is able to capture solar radio burst (SRB) events with a signal-to-noise ratio of more than 15 dB in the case of the brightest bursts [1][2]. These signals are of great value to the heliophysics community for a number of reasons: (i) the very high temporal and frequency resolutions of SHARAD, (ii) its position in the solar system, in Martian orbit at 1.5 AU, and (iii) its sensitivity in the 13.3–26.7 MHz band completes the frequency chain between typical dedicated solar missions and Earth radiotelescopes. Dedicated space-based solar radio-observatories include the WAVES instrument, a version of which is present on both the Wind (1.075–13.825 MHz) and the STEREO spacecraft (0.125–16.075 MHz).

We argue that these results can also be useful for radar science itself. An issue usually encountered during the conception of radar sounders is the impracticality to provide an absolute calibration for the instrument. As a consequence, most radar sounders including SHARAD provide uncalibrated data, in the form of dB above an unspecified background power level. Several post hoc calibration schemes have been suggested for SHARAD and MARSIS [3][4], relying on assumption on the roughness and dielectric composition of certain Martian terrains.

In this abstract, we propose a radar sounder calibration method based on correlated detections of type III solar bursts. In [1] we presented 38 solar bursts detected by SHARAD and at least one of the three WAVES-carrying spacecraft. STEREO/WAVES being a calibrated instrument that shares part of its with that of SHARAD, we can compare individual bursts one-to-one within their common frequency bands. Considering all bursts with an equal weight, and considering the integrated power between 13.3 MHz and 16 MHz, we obtained that

SHARADdB = 0.015 STEREOdB[W/m²] + 12.82,

although with significant spread (r2 = 0.018). For more accuracy, the directivity of the burst (i.e., its intensity variations both in radial and longitudinal directions) should be taken into account, as well as the specific gain of SHARAD in the direction of the Sun at the time of detection.

Acknowledgements

Part of this research was carried out at the JPL, California Institute of Technology, under a contract with NASA.

References

[1] Gerekos, C., et al., AGU Fall Meeting Abstracts, SH22E-2042, 2022

[2] Gerekos, C., et al., arXiv:2307.01747, June 2023.

[3] Mouginot, J., et al. Icarus 210.2 (2010): 612-625.

[4] Campbell, Bruce A., et al. Icarus 360 (2021): 114358.

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